1. Field of the Invention
The present invention relates to a die cushion force control method and a die cushion apparatus, and more particularly, to a technique for suppressing vibration of die cushion force at the start of die cushion force control.
2. Description of the Related Art
Conventionally, in controlling a die cushion force using a servo die cushion apparatus configured with a servomotor or servo valve, a vibration problem occurs due to excessive response at the point when a slide of a press machine and a cushion pad impact against each other through a die (upper die), a blank, a blank holder, cushion pins, and the like in this order, starting die cushion force control, where the vibration problem lies in that a die cushion force becomes larger (overshoot) or smaller (undershoot) than a die cushion force command value (set force).
It has been considered that the vibration problem occurs depending on performance and load conditions of the die cushion apparatus and in most cases, the vibration problem has been considered to be unavoidable (with resignation). Consequently, there are often such problems as blank breakage caused by overshoot or wrinkles caused by undershoot.
Japanese Patent Laid-Open No. 2010-188363 describes a servo die cushion control apparatus which causes a die cushion force vibrating after an impact between a slide and a cushion pad to converge quickly to a desired value. In order to suppress an undershoot occurring after an initial large overshoot, a next recurring undershoot smaller than the initial undershoot, and the like, the servo die cushion control apparatus is designed to create a second force command value which decreases to a set die cushion force (first force command value) using a force value detected when a local maximum point of the initial overshoot is reached as an initial value and switch the force command value from the first force command value to the second force command value when the local maximum point of the initial overshoot is reached. That is, under conditions in which an overshoot occurs at the time of an impact between the slide and die cushion, responsiveness of the force after the overshoot is improved by switching the force command value around the time of arrival at a local maximum point of the detected force value.